Nucleation of Hybrid Polymers in Sickle Cell Disease

Upon deoxygenation, sickle hemoglobin (HbS) can polymerize into complex, 14 stranded polymers via a double nucleation process. While sickle cell hemoglobin nucleation has been well described, the description has focused on nucleation of a single species, deoxyHbS, with perhaps the presence of a crowding non-polymerizing species. However, there are three important cases of polymerization where hybrid polymers are created, and thus the nucleation process in such situations needs to be characterized and tested. First in vivo, polymerization always occurs in the presence of ligands. Second, polymerization is also possible in the presence of normal hemoglobin (HbA, eg. in sickle trait). Finally, antisickling drugs may not bind to all molecules and thus create a heterogeneous population with the opportunity for hybrid polymerization. Thus we have studied polymerization in the presence of HbA, as well as under cases of partial ligation (with CO, NO and O2). Homogeneous nucleation rates have been measured by laser photolysis of the CO-derivative and analysis of the stochastic fluctuations of onset-times. Heterogeneous nucleation is determined by following the exponential growth of light scattering. Existing models for nucleation have been successfully modified to account for the copolymerization probability of hybrid species, and revised models will also be presented. Incorporation of ligands is particularly challenging since it appears necessary to account for tertiary as well as quaternary effects, and these appear to differ depending on the ligands.